Method of and apparatus for electrolytic refining



April 2s, 1925. 1,535,458

F. C. FRARY METHOD OF AND 'APPARATUS FOR ELECTROLYTIC REFINING FiledNov. 5, 1925 6mp/life INVENTOR Eme/'5 C Ffa/y ATTORNEY Patented Apr. 28,1925.

yUNITED STATES 1,535,458 PATENT OFFICE.-

FRANCIS C. FRARY, F OAKMONT, PENNSYLVANIA, ASSIGNOR T0 ALUMINUM COMfPANY 0F AMERICA, OF PITTSBURGH, PENNSYLVANIA, A CORPORATION 0F PENN-SYLVANIA.

i METHOD 0F AND,APPARATUS FOR ELECTROLYTC REFINING.

Application filed November To ZZ tlf/mm, it may @mwa/"n:

Be it known that I, FRANCIS C. Farini', a citizen of the United Statesof America, residing at Oakmont, in the county of Allegheny and State ofPennsylvania, have invented certain new and useful Improvements inMethods of and Apparatus for Electrolytic Refining, of which thefollowing is a full, clear, and exact description.

In the electrolytic refining of aluminum by methods such as described inthe copending applications (all filed December 2l. 1922) of WilliamHoopes, Serial No. 608,287; lVilliam Hoopes, Junius D. Edwards andmyself, Serial Nos. 608,284 and 608,285; William Hoopes and myself,Serial No. 608,286, and in similar methods in which a layer of moltenmetal as cathode floats on a bath of molten electrolyte containing asalt of an alkali metal, such as sodium ilnorid, great difficulty hasbeen experienced in making satisfactory electrical connection betweenthe cathode and the external circuit. So far as I am aware, none of themethods 25 heretofore proposed is wholly successful in practice. Forexample, metal electrodes are attacked and dissolved by the moltenmetal, especially in the Case of aluminum. Tater-cooled metallicelectrodes (preferably iron) covered with a thin layer or coating offrozen electrolyte, such as described in United States patent of vonKugelen and Seward, No. 858,400, can be used, but such devices possesscertain disadvantages because of the high contact resistance between themetal of the electrode and the metal of the cathode, causing greaterpower consumption. This is further increased by the loss of the heatcarried away by the cool ing water. Such factors as these seriouslyaffect the efficiency of the process. Moreover, there is always dangerof the protective coating being penetrated by the molten metal, whichmay dissolve and alloy with the iron and thus produce a leak, to saynothing of contaminating the cathode. This is especially liable to occurif anything happens to produce an are at any point between the electrodeand the molten metal. Electrodes of amorphous carbon, such, for example,as are employed as anodes in the electrolytic reduction of alumina andfor leading current into'ordinary electric furnaces, have been foundwholly unsuitable 5, 1923. serial No. 672,867.

for my purpose, for th\e reason that when used as cathode connections ina process of the kind referred to they undergo a peculiar and rapiddestruction, due apparently to the action of alkali metal or vapor uponthe carbon connector or upon the binding material therein. Under somecircumstances such carbons, if in contact with the molten bath', willflake ofi' in thin flakes, say about an eighth of an inch in thickness,so rapidly that the carbon will be rendered completely useless in a fewminutes. Under other circumstances the carbon connector, particularlywhere it is in contact with a relatively small amount of the bath suchas may work up' through the layer of moltenmetal, is subject to a rapiddisintegration which causes it to swell up and become. simply a mass ofloosely adhering particles. y

In the course of extended experience with the refining processesdescribed in the above mentioned applications, in which the b-ath orelectrolyte contains `fluorids of aluminum, sodium, and an alkalineearth metal such as barium, I have found that connectors made ofgraphite serve admirably, and indeed are the only ones suitable for thepurpose., Such connectors may be made by graphitizing amorphous carbonrods at such a high temperature that all or substantially all of the ashis volatilized and the carbon converted into graphite. Such electrodesor connectors are also attacked in some degree by sodium liberated undersome conditions in the electrolysis, but they are so resistant that theattack isv very slow, and unde-r most circumstances they will lastseveral days, while if care is taken to maintain favoi-able bathconditions, for example by the use of excess aluminum fluorid, asdescribed` in the above mentioned application of William Hoopes, JuniusD. Edwards and myself, Serial No. 608,285, they often show but slightdeterioration after a week or more of continuous use.

The use of graphite electrodes' or connectors in a process of the kindreferred to is illustrated in the accompanying drawing, in which theligure is a diagrammatic cross section showing the anode, the bath orelectrolyte, and the molten aluminum cathode. with graphite connectorsin the form of short rods dipping into the latter.

CTI

In the drawing, 10 represents a layer-of molten aluminum alloy as anode,which may contain copper and silicon, as for example the alloy describedin the above mentioned copending application of William I-Ioopes, JuniusD. Edwards and myself, Serial No. ($08,284. This layer is shown restingon a conducting hed of carbon, 11, which may be the bottom lining of thecell in which the electrolytic. refining process is carried out. Thecarbon bed serves as an electrode by which the anode allov is connectedto the positive terminal of the external circuit, represented by theconductor 12.

Floating on the molten anode alloy is a layer 13 of molten electrolyte,which may be of about the following composition, as described in theabove mentioned copending application of William Hooes, Junius D.

Edwards and myself, Serial o. 687,285:

Barium uorid 30 to 38 per cent Sodium fiuorid -25 to 30 per centAluminum fluorid -30 to 38 per cent Alumina 0.5 to 5 per cent Calciumand magnesium fiuorids (present as unavoldable impur1t1es) about 2 percent vFloating on the electrolyte is the layer 14 of molten aluminumconstituting the cathode.

The graphite connectors 15, dipping into the molten cathode 14, arepreferably in the lform of short, thick cylinders7 about six inches longand from four to six inches in diameter. Any suitable means may beemployed to connect them to the negative terminal of the externalcircuit1 represented by the busbar 16. In the drawing copper rods 17,screwed into the tops of the connectors, are provided for the purpose.These rods are clamped or otherwise secured to the busbar in anyconvenient manner, and may serve to support or suspend the graphiteconnectors therefrom.

At 18 is shown a layer or crust composed largely of electrolyte workingup through or around the molten cathode metal and solidifying above thesame'.

As before stated, the connectors may be made by graphitizing amorphouscarbon at high temperature, so as to volatilize the ash and convert thecarbon into graphite. This treatment may be performed upon carbon rodsof suitable length, from which the connectors may be out afterwards.

The present invention is disclosed inthe copending application ofWilliam Hoopes, Ser. No. 608,287, and is stated therein to be thesubject 'of my present application.

It is to be understood that the invention is not limited to the detailsherein specifically set forth but can be carried ont in other wayswithout departure from its spirit as defined by the appended claims.

I claim:

1. In a process of electrolyzing a fused electrolyte containing afiuorid of an alkali metal, in which a layer of molten metal floating onthe electrolyte acts as cathode, conveying electrolyzing current betweenthe cathode and the external circuit through a connector the lower endof which is composed essentially of graphite in contact with said layerof molten metal.

2. In a process of electrolytic refining of aluminum, in which a cathodelayer of inolten aluminum floats on a bath or electrolyte containingcryolite, conveying the electrolyzing current between the cathode andthe external circuit through a connector having its lower end composedessentially of graphite in contact with the layer of molten aluminum.

8. In a process ol' refining aluminum electrolytically, in which acathode layer of molten aluminum floats on a bath or electrolytecontaining sodium fiuorid and a fluorid of an alkaline earth metal,conveying electrolyzing current between the cathode and the externalcircuit through a connector having an end composed essentially ofgraphite.

4. In an apparatus for electrolytically refining aluminum with anelectrolyte containing alkali metal fluorid, and a cathode of moltenaluminum floating thereon, au electrical connector between the externalcircuit and the cathode, comprising a body of graphite adapted to 'dipinto the molten cathode, and a conductor in electrical contact with thegraphite body.

5. In an apparatus for electrolytically refining aluminum with anelectrolyte containing all-:ali metal fluorid, and a cathode of moltenaluminum floating thereon, an electrical connector between the externalcircuit and the cathode, comprising a body of graphite adapted to dipinto the molten cathode, and a metallic conductor having one endembedded in the graphite body.

G. In an apparatus for electrolytic refining of aluminum, a moltenelectrolyte containing an alkali metal fluorid, a molten aluminumcathode floating on the molten electrolyte, an external circuit hav'ingits positive pole connected with the electrolyte, and a connectorelectrically connected with the negative pole oi' the external circuitand having ali-end composed of graphite in direct contact with themolten aluminum cathode.

In testimony whereof I hereto affix my signature.

FRANCIS C. FRARY.

